In the manufacture of paper on continuous papermaking machines, a web of paper is formed from an aqueous suspension of fibers (stock) on a traveling mesh papermaking fabric as water drains by gravity and suction through the fabric. The web is then transferred to the pressing section where more water is removed by pressure and vacuum. The web next enters the dryer section where steam heated dryers and hot air completes the drying process. The papermaking machine is, in essence, a water removal, system.
Conventional papermaking machinery includes equipment to set the sheet properties of the paper as it is being manufactured. One of the more basic operations on a papermaking machine is control of the cross-direction (CD) moisture profile by rewetting or remoisturizing with water sprays administered by spray nozzles. By applying water to the drier areas of a sheet, a uniform CD moisture profile can be created. This is usually carried out to ensure that the moisture profile at the reel is uniform. Similarly, steam heating of a paper sheet is widely practiced in papermaking. The increase in sheet temperature that results provide increased drainage rates for the water thus reducing the amount of water to be evaporated in the dryer section. Water drainage is improved by the application of steam principally because the heating of the sheet reduces the viscosity of the water, thus increasing the ability of the water to flow. Most of the heat transfer takes place when the steam condenses in the sheet. The condensation of the steam transforms the latent heat of the steam to sensible heat in the water contained by the sheet. As in the case of rewetting, in steam heating the amount of steam applied may be varied across the width of the sheet along the cross machine direction so that the CD moisture profile of the sheet may be modified.
Unfortunately, in both rewet and steam shower processes, water tends to accumulate in the vicinity adjacent the points of application. Large water droplets develop on surrounding structures that can result in the formation of rust and related problems. In addition, the rotating rolls and airflow of the machinery can propel droplets onto the paper sheet. Conventional vacuum boxes, which are not energy efficient, demonstrate only limited success in removing the excess steam and moisture; as a result, the excess water is detrimental to the production of high quality paper and the operations of the machinery.